North Pacific subtropical sea surface temperature frontogenesis and its connection with the atmosphere above
The net heat flux and meridional temperature advection in the ocean are two factors in the North Pacific subtropical sea surface temperature front (NPSTF) frontogenesis occurring from October to the following February. However, the relative importance of these two factors has been rarely explored. In this study, frontogenesis of the NPSTF is examined quantitatively based on the mixed-layer heat budget equation to clarify the relative importance of net heat flux and meridional temperature advection and to further explore its connection with the atmosphere above. Diagnosis results show that the net heat flux dominates the frontogenesis from October to December, while the meridional temperature advection in the ocean contributes equally as or even more than the net heat flux in January and February. The atmosphere is critical to the frontogenesis of the NPSTF, including the direct effect of the net heat flux and the indirect effect through the Aleutian low. Further analyses demonstrate that the latent heat flux (the shortwave radiation) dominates the net heat flux in October (from November to February). The meridional temperature advection in the ocean is mostly due to the meridional Ekman convergence, which is related to the Aleutian low. Climatologically, the strengthening and southward migration of the Aleutian low from October to the following February are characterized by the acceleration and southward shift of the westerly wind to the south, respectively, which can drive southward ocean currents. Correspondingly, the southward ocean currents provide for colder meridional advection to the north of the NPSTF in January and February, favoring frontogenesis. In addition, the Aleutian low plays a role in transforming the dominant effect of the net heat flux into the joint effect of meridional temperature advection and net heat flux in January.